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Surface & Coatings Technology

Effect of rare earth (Ce, La) compounds in the electroless bath on the platingrate, bath stability and microstructure of the nickel–phosphorus deposits

H. Ashassi-Sorkhabi ⁎, M. Moradi-Haghighi, M.G. Hosseini

Electrochemistry Researches Laboratory, Department of Physical Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran

Received 27 May 2007; accepted in revised form 11 July 2007Available online 20 July 2007

Abstract

Effects of added rare earth elements (RE) in the acidic hypophosphite plating bath on the plating rate, bath stability and microstructure of theelectroless nickel–phosphorus (EN) deposits were studied. The surface appearance and microstructure were examined under a reflection opticalmicroscope and a scanning electron microscope equipped with an in-situ energy dispersive X-ray spectroscopy, which can evaluate the elementalanalysis of deposits. It was demonstrated that the rare earth elements can decrease grain size and refine microstructure.

The deposition rate of the Ni–P deposits was estimated by gravimetric, polarization and quartz crystal microbalance (QCM) methods. Resultsrevealed that up to an optimum concentration of rare earth elements, the deposition rate increases. The stability test method was used to determinethe stabilization effect of RE on the stability of the bath. It was found that the addition of RE significantly improved the Pd stability of the EN bath.© 2007 Elsevier B.V. All rights reserved.

Keywords: Electroless nickel plating; Rare earth elements; Deposition rate; Bath stability; Quartz crystal microbalance

1. Introduction

Since the discovery of electroless or autocatalytic nickelplating, it has been widely used in electronics machinery,automobile, aerospace and other industries [1,2]. With excellentproperties such as non-magnetic, low internal stress and highcorrosion resistance, Ni–P alloy coating with high-P content hasbeen an important undercoat for computer hard disks [3].

Rare earth elements have many special properties, such asmagnetic, optical, and electric and hydrogen storage propertiesand have been successfully used in many fields such asmetallurgy, electronics and chemical engineering [4,5].Researchers have found that the rare earth elements caninfluence on certain parameters like deposition rate and stabilityof the bath in Cr, Ni and Cu electroplating process, [6–10]. It isalso reported that these elements improve mechanical propertiesof alloys, like tensile strength, toughness and fatigue resistancein alloys such as Al–Li and Al–Si [11].

⁎ Corresponding author. Tel.: +98 411 3393136; fax: +98 411 3340191.E-mail addresses: habib_ashassi@yahoo.com, ashassi@tabrizu.ac.ir

(H. Ashassi-Sorkhabi).

0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2007.07.019

In the present study, the effects of added rare earth elements(RE) in acidic hypophosphite plating bath on the properties ofthe resulting electroless Ni–P deposits were studied. Theauthors found that the addition of rare earth elements canincrease the plating rate and bath stability, and improve the filmmicrostructure by refining the grains of deposits.

2. Experimental details

2.1. Materials and reagents

The gold plated quartz crystals and mild steel coupons wereused as the substrates. The chemicals used in the experiments, allpurchased fromMerck, were of reagent grade and usedwithout anyfurther purification. The used EN solution bath was formulated asTable 1. It was chosen because a glycin–citrate combinationproduces a complex with nickel and makes the plating solutionstable [12]. The initial pH value of the plating bath was adjusted bydilute NaOH solutions to 4.5±0.1. De-ionized water was used forsolutions preparation and rinsing of glassware. Using a digitallycontrolled thermostat (Memert), the bath temperature wascontrolled within 85±1 °C under atmospheric environment. Theplating was conducted by immersing one piece of the substrate intoEN solution (100 ml) in a 150 ml beaker for 1 h.

Table 1Bath composition and deposition conditions

Composition Condition

NiSO4·6H2O 30 g dm−3 pH 4.5±0.1NaH2PO2·H2O 25 g dm−3 t 85±1 °CCH3COONa 20 g dm−3

H2NCH2COOH 20 g dm−3

Ce (IV) Sulfate, La nitrate 0–1 g cm−3

Pb (NO3)2 2 mg dm−3

NaOH for adjusting pH

1616 H. Ashassi-Sorkhabi et al. / Surface & Coatings Technology 202 (2008) 1615–1620

2.2. Apparatus and experimental procedures

Three methods were used for plating rate measurements.These were gravimetrical, polarization and QCM methods.

2.2.1. Gravimetrical methodIn this study, the mild steel coupons (99.47% Fe) were used

as the substrates. The steel coupons were polished with theabrasive papers from 400 to 1000 grade, cleaned in concentratedNaOH and activated by acid dip in 30% HCl. The plating rate(R: mg cm−2 h−1) of the electroless Ni–P alloy was determinedby gravimetrical method and was expressed in terms of theweight gain during the deposition process. An analyticalbalance (Unimatic CLX40) with a precision of 0.1 mg wasemployed to weigh the as-deposited samples. The plating ratewas calculated according to the following formula: where t isthe plating duration (min), Mt (mg) is the mass of the objectplated for a length of time, M0 (mg) is the initial weight ofsubstrate and As is the surface area of specimen (cm2) [13].

R ¼ Mt �M0ð Þ � 3600As � t

:

Table 2Measured parameters for different concentration of cerium and lanthanum

Experimentalmethod

Gravimetric Polarization QCM

T=85 °C T=85 °C T=50 °C

Substrate Rate idep Rp Rate Rate

(mg cm−2

hr−1)(mA cm−2) (ohm) (mg cm−2 h−1)

Nickel 13.6 1.73 23.47 1.85 0.56

2.2.2. Polarization measurementsThe electrochemical measurements were carried out using an

electronic potentiostat (Autolab PG-stat 30). The workingelectrode was freshly electroless nickel deposited iron platedsized 1 by 1 cm. The auxiliary electrode was a platinumelectrode and a saturated calomel electrode (SCE) used as areference electrode. Linear scanning voltammetry (LSV) wasused to plot anodic and cathodic polarization curves. The scanrate was 1 mV/s. According to the steady state equilibriumpotential, the rate of reduction (deposition) of metal is equal tothe rate of oxidation of reducing agent i.e. [14]:

idep ¼ im ¼ ired:

Ce 3 ppm 18.3 2.76 18.50 3.01 0.56Ce 6 ppm 22.6 2.84 18.79 3.09 0.56Ce 8 ppm 20.1 2.10 18.85 2.28 0.56Ce 10 ppm 18.4 2.08 18.70 2.27 0.56Ce 20 ppm 18.0 1.61 27.03 1.75 0.56Ce 200 ppm 16.5 – – – 0.00Ce 1000 ppm 7.9 – – – 0.00La 2 ppm 19.4 2.48 18.91 2.70 0.56La 4 ppm 18.2 2.87 30.23 3.13 0.56La 5 ppm 19.0 2.43 18.03 2.65 0.56La 10 ppm 22.4 2.89 17.35 3.15 0.56La 20 ppm 17.10 – – – 0.56

2.2.3. QCM methodQuartz crystal microbalance studies were performed using a

Maxtec PM-710 plating monitor coupled with a MPS-550sensor probe, the equipment is calibrated automatically. TheMaxtec quartz resonators were made from AT-cut quartzcrystals (resonance frequency, 5 MHz having a resolution ofthe order of 0.5 Hz, which translates to a thickness resolution of10 ng cm K2) covered by evaporated gold on both faces(apparent electrode areas, 0.316 cm2 for the “small” side,

1.37 cm2 for the “large” side). The quartz crystal wasincorporated in a sensor probe made of PTFE.

One O-ring ensures that the liquid comes in contact with oneside of crystal only (large side). The oscillator circuit washoused in the probe head. Frequency changes were converted tomass loading using the Sauerbrey formula [15,16].

2.2.4. Bath stability testThe Palladium stability test method was used to determine

the effect of rare earth elements on the stability of the bath. Thistest was directed at the lower temperature of 65±1 °C toamplify the difference in stabilities of the plating solutions.When the solution temperature reached to 65±1 °C, 5 ml of40 mg/l PdCl2 was added to the solution and the time requiredfor the solution to be decomposed was recorded. The end pointor the onset of bath decomposition was defined as the time whenthe solution became dark green and opaque [17].

2.2.5. Surface analysisThe surface morphology of the Ni–P deposits was

investigated using optical microscope at low magnitude(×100) in reflected mode and the scanning electron microscopy(LEO 440i). Energy dispersive X-ray spectroscopy (EDAX)with a detection limit of ∼0.5 wt.% for most elements was usedto analyze the Ni and P content (wt.%) of the Ni–P deposits.

3. Results and discussion

3.1. The effect of rare earth elements on the plating rate ofelectroless Ni–P

Table 2 shows the rates of electroless Ni–P depositionobtained from gravimetrical and polarization methods in thepresence of various concentrations of rare earth compounds. Itdisplays that the additives in the plating bath are able to increaseplating rate in a range of optimum concentrations.

Fig. 1. Polarization curves of Ni–P deposits without and with RE as additives.

Fig. 2. QCM curves for the effect of cerium and lanthanum on the electrolessnickel-plating rate.

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Many factors can affect the deposition rates of the electrolessNi–P alloys. For example, temperature, pH of solutions, tankloading, concentration of nickel and the reducing agent and thesurface properties of the substrate [18]. So it is demonstrated thatsurfactants [18] and stabilizers [13,19] can alter the deposition rate.

Rare earth elements can also affect the plating rate for thefollowing reasons:

1. The Ce and La are surface active elements with a rather largeatomic radius (radius of cerium and lanthanum are 0.1824and 0.1877 nm respectively) and low electronegativities(1.05 and 1.1 for Ce and La respectively). It means that theCe and La form positive ions easily. They can increase thenumber of crystal nuclei and hinder the growth of grains Ni–P deposition. Furthermore, the Ce and La can reduce surfacetension and interfacial energy; thereby decrease criticalnucleation work because of their chemical activity. All ofthese factors result in an increase in nucleation rate, thus themicrostructure of the coatings can be refined [5].

2. The overall reaction of electroless Ni–P deposition in acidichypophosphite bath can be broadly expressed as [20]:

2H2PO�2 þ Ni2þ þ 2H2OY2H2PO

�3 þ H2 þ 2Hþ þ Ni0:

Hydrogen is one of the products of the reaction and beadsorbed on the surface of as-deposited substrate to preventfurther deposition of nickel on sites where the bubblesoccupy. RE elements can reduce the interfacial tensionsbetween the generated hydrogen bubble and the as-depositedsubstrate and remove hydrogen gas on the as-depositedsubstrate quickly. Thus they may shift reactions to thedirection in favor of the reduction of Ni2+ ions and increasingthe deposition rate.

3. These elements lower the activation energy of platingreaction, therefore they increase deposition rate up to REconcentration that is less than 100 ppm. A higherconcentration of RE becomes poisonous to the depositiondue to the surface coverage of the deposit by RE elements.The poisoning effect of the surface, in the case of high REconcentration, overcomes to the lowering effect on theactivation energy. It is important to determine the effectiveconcentration of these additives for specific bath chemistry.

The decrease in deposition rate at higher concentration is dueto the reduction in the number of catalytic sites on the metalsurface because of the formation of thin film of adsorbed RE.This reduces the possibility for interaction of hypophosphitewith the catalytic sites, resulting in the inhibition of theoxidation of hypophosphite and thereby the poisoning of theoverall reaction.

Fig. 1 shows the linear polarization curves for the electrolessNi–P plating in the absence and presence of the rare earthelements additives. The obtained deposition currents (idep) wereused to calculate the deposition rates. As can be seen fromTable 2,the same trend exists for the effect of rare earth elements on thedeposition rates as the gravimetrical method, but the ratescalculated from Idep are lower. In fact the electrochemicalmechanism is not operative in the case of Ni–P systems and thedeposition process is mainly due to predominance of a chemicalmechanism [21]. Nickel ion reduction in the EN solution in thebeginning of the electroless deposition is known to be controlledby the anodic processes, and the first step involves a non-Faradicstep, namely the adsorption of hypophosphite on the catalyticsurfaces [17].

In electrochemical method, the reduction of nickel andoxidation of hypophosphite is caused by applied potentialmaking double layer around the anode and cathode and thismakes the process a two-step reaction for the charge transfer,while in gravimetric method the charge transfer betweenoxidation and reduction species takes place directly, thus inthis case the process is faster [21].

Fig. 2 shows QCM curves of plating rate of electroless Ni–Pin the presence of rare earth compounds in the plating bath.These curves show that the RE cannot increase considerably theplating rate. Therefore, it can be deduced that the effect of REon plating rate depends on type of substrate. According to theatomic hydrogen theory of the electroless nickel-plating,tetrahedral hypophosphite is adsorbed on the catalytic metalsurface, the hydrogen atom of P–H bond locates on the metalsurface. Due to the higher electronegativity of the oxygen atom,the phosphorus atom is positively charged. Consequently, under

Table 3Rate of hydrogen evolution (2H++2e→H2) for different substrate [19]

Electrode Fe Ni Pt

i (A cm−2) 0.01 0.06 8

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the direct attack of hydroxide ions, the cleavage of the P–Hbond happened to produce a hydrogen atom at the catalyticsites, which results in the subsequent hydrogen evolution andnickel deposition that decreases the free nickel ions and platingrate [22]. In QCM method, gold electrode was employed and itwas a suitable base for evaluation of hydrogen. Hydrogenmolecules adsorb on catalytic surface very fast and as theinterfacial tension is very high, the RE cannot remove producedhydrogen gas on as-deposited surface quickly. In fact, theelectrode materials with high excess hydrogen potential are thebest choices [23], i.e. when the substrate is nickel or steel, thereduction of cerium overcomes to the hydrogen evolution andshows its effect. Table 3 demonstrates the rate of hydrogenevolution on different substrate [24].

3.2. Surface morphology and elemental analysis

The properties of electroless Ni–P deposits are attributed totheir microstructural characteristics. The details of structure ofEN deposits are not well understood but as plated EN films havebeen reported to be either crystalline, amorphous or a co-existence of both. Studying the microstructure of the depositshelps us to understand the mechanism of deposition andevaluate the properties of EN deposits [20]. It is well-known

Fig. 3. Optical microscopy images of electroless nickel with different co

that the addition of Ce in small amounts to engineering alloyssignificantly modifies the surface passive layer [25]. Fig. 3shows the surface appearance of the deposits without and withCe compound in the bath, observed by optical microscopy. Thedeposits show differences in morphology when deposited fromthe baths containing different concentration of cerium. Thesurfaces of Ni–P deposit with RE are very uniform andmicrocrystalline. In fact, the grains of deposit obtained from thebath containing RE element additive are finer than the coatingwithout RE. This indicates that the addition of Ce influenced thenature of surface film. The refining effect of the RE makes themicrostructure more compact, because when the RE concen-tration is too high, for example 1000 ppm (1 kg m−3), the grainsize becomes larger. Therefore, it should be emphasized that it issuitable to add a small amount of Ce and La to the plating baths.The same results could be deduced from SEM images. In Fig. 4,it is observed that Ce 10 ppm adding resulted in a finer grainsize material, uniformed coating and smooth and mirror-likemicrographs. In fact, an increasing amount of RE elements up tooptimum gives rise to amorphous structure; but the higherconcentration (1000 ppm) of RE causes inhomogeneity ofdeposits and reduces the quality of structures. Also the whitespots observed on the surface indicate the presence of cerium inthe Ni–P coating.

The composition of the Ni–P deposits measured by EDSwithout and with different concentration of cerium salt in thebaths is shown in Table 4. We can find out that the phosphorouscontent in the Ni–P film reduces with bath Ce salt content.According to the following reactions [17,26], the phosphorousatoms were formed by electron transfer between nickel and

ncentration of cerium: A: 0 ppm, B: 4 ppm, C: 10 ppm, D: 15 ppm.

Fig. 4. SEM images of electroless nickel with different concentration of cerium: A: 0 ppm, B: 3 ppm, C: 10 ppm, D: 20 ppm, E: 100 ppm.

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hydrogen atoms and hypophosphite ions. The high concentra-tion of rare earth elements blocked the catalytic sites at thecatalytic surface and reduces the phosphorus production.

Nicat þ H2PO�2 YPþ NiOHad: þ OH�

H on catalytic surfaceð Þ þ H2PO�2 YPþ H2Oþ OH�

Cerium can also co-deposit with Ni–P and probably it mayhold at grain boundary area. For this reason, the EDS analysiscannot recognize it in low concentration, but in high concentrationof Ce salt in the bath the EDS analysis shows the Ce in deposits.

Table 4Effects of RE concentration on phosphorous content of electroless nickel plating

Substrate P content (wt.%)

Electroless Ni–P 11.35EN with Ce 10 ppm 10.61EN with Ce 20 ppm 9.37EN with Ce 1000 ppm 9.85EN with La 10 ppm 10.34

3.3. Bath stability analysis

Sudden bath decomposition can result in an increase in costsand the production of environmentally hazardous pollutants dueto the large waste generation [17]. The palladium stability testshowed a significant increase in bath stability with the addition

of either cerium or lanthanum. For Ce and La, the bath lifeincreased from 20 min to 80 and 95 min respectively when thestabilizer concentration was 10 ppm.

The hypophosphite is oxidized during the electroless nickeldeposition reaction to form nickel phosphite. Nickel phosphitemay precipitate at high temperatures and result in bathdecomposition. The stabilizers adsorb on the nickel phosphitenuclei and inhibit the decomposition reaction. However, moreinformation is available regarding to the effect of these

1620 H. Ashassi-Sorkhabi et al. / Surface & Coatings Technology 202 (2008) 1615–1620

stabilizers on the growth and properties of the deposit [25]. Therare earth elements in the electroless nickel bath stabilize it bypoisoning the catalytic activity of the accidentally formed nickelphosphite or metal hydroxide nuclei. It was expected that theywould also adsorb onto the deposit. Too much adsorption of theRE on the substrate or deposit certainly would also poison thedeposition reaction.

4. Conclusion

1. An addition of optimum amounts of rare earth elements tothe plating baths could increase the deposition rate incomparison with the RE-free bath.

2. The substrate material can alter the RE elements effects. Assubstrate is gold, the rate of hydrogen evolution becomes sohigh therefore, the existing RE cannot show its effect.

3. In high concentrations of RE compound, the rare earthelements adsorb on the surface of the Ni–P deposits anddecrease the deposition rate of EN plating and the homoge-neity of the coats.

4. Ce and La refine the microstructure of EN deposit andproduce smooth and mirror-like coatings.

5. The phosphorus contents of the resulting Ni–P depositsdecrease slightly with addition of RE compounds in the baths.

6. The addition of RE elements significantly improves the Pdstability of the EN bath.

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